In a construction machine such as hydraulic pressure shovels, etc., a plurality of work devices and upper revolving bodies such as booms, arms, and buckets are provided, and these plurality of work devices and upper revolving bodies are variously driven and operated by corresponding a plurality of hydraulic actuators (hydraulic pressure cylinders, hydraulic pressure motors).
Normally, a plurality of (2 units) variable displacement hydraulic pumps, specifically first and second hydraulic pumps, are used as the drive source of these plurality of hydraulic actuators.
Pressure oil is supplied from the first hydraulic pump to a first main operation valve through a first discharge fluid passage, and the pressure oil that has passed through the first main operation valve is supplied to the first hydraulic actuator. Here, the first main operation valve is manipulated by an operation lever, for example, on the left side. The left operation lever is an operation lever that operates the action, for example, of an arm and an upper rotating body, and the first hydraulic actuator is a hydraulic actuator for a work device that operates the arm and the upper and rotating body. By manipulating the left operation lever, a direction and a flow rate of the pressure oil supplied from the first main operation valve to the first hydraulic actuator is changed, and the arm and the upper rotating body is operated in a direction and at a velocity corresponding to this.
Meanwhile, pressure oil is supplied from a second hydraulic pump to a second main operation valve though a second discharge fluid passage, and the pressure oil which has passed through the second main operation valve is supplied to a second hydraulic actuator. Here, the second main operation valve is manipulated, for example, by an operation lever on the right side. The right operation lever is an operation lever that manipulates, for example, the operation of a boom and a bucket, and the second hydraulic actuator is the hydraulic actuator for the work device that operates the boom and the bucket. By manipulating the right operation lever, a direction and a flow rate of the pressure oil supplied from the second main operation valve to the second hydraulic actuator is changed, and the boom and the bucket is operated in a direction and at a velocity corresponding to this.
Patent literatures 1, 2, and 3 described later are inventions in which the hydraulic pressure circuit of the construction machine is provided with a merging/separating valve which sets the first discharge fluid passage and the second discharge fluid passage in a connected state or a blocked state, and the merging/separating valve can be switched between the merge position and the separation position. When changing the merging/separating valve to the merge position, the first discharge fluid passage and the second discharge fluid passage are connected, and both discharge fluid passages enter the merge state; when changing the merging/separating valve to the separation position, the first discharge fluid passage and the second discharge fluid passage are blocked and enter the separation state.
With a construction machine, there are many opportunities to perform operations by simultaneously manipulating a left and right operation levers, driving the first and the second hydraulic actuators simultaneously, and thereby conducting complex operations of a plurality of work devices corresponding to the respective first and second hydraulic actuators.
Here, when simply merging the first discharge fluid passage and the second discharge fluid passage and simultaneously driving a plurality of hydraulic actuators, even if the left and right operation levers are manipulated only the same amount, the hydraulic actuator with the smaller load (for example, the first hydraulic actuator) is supplied at a large flow rate, the hydraulic actuator with the larger load (for example, the second hydraulic actuator) is supplied at a small flow rate, and there is loss of operability.
Then, every first and second main operation valves is provided with a first and second pressure compensation valve so that the flow rate corresponding to the amount of operation of the left and right operation levers is fed to the first and the second hydraulic actuators.
When changing the merging/separating valve to the merge position, pressure is compensated by the first and the second pressure compensation valves at the same time. Pressure is compensated by introducing into the first and the second pressure compensation valves the maximum load pressure, for example, P2, from among the load pressures P1 and P2 of the first and the second hydraulic actuators. In addition, when switching the merging/separating valve from the merge position to the separation position, the pressure compensation based on the first and the second pressure compensation valves is simultaneously released. The pressure compensation is released by introducing to the respective first and second pressure compensation valves the load pressure of the hydraulic actuator itself, rather than the maximum load pressure.
Letting the open area of the first and the second main operation valves be A1 and A2; the differential pressure before and after narrowing the first and the second main operation valves be ΔP1 and ΔP2; and the flow rate coefficient be c, the pressure oil flow rates Q1 and Q2 (L/min) supplied to the first and the second hydraulic actuators from the first and the second main operation valves are expressed in the following formulae (1) and (2):Q1=c·A1·√(ΔP1)  (1)Q2=c·A2·√(ΔP2)  (2)
When pressure compensation is performed, the differential pressure before and after narrowing the first main operation valve on the light load side, namely, ΔP1 of the right side of the aforementioned formula (1), is the same value as differential pressure before and after narrowing the second main operation valve on the heavy load side, ΔP2. For this reason, in the pressure compensation state, the relationship indicated in the following formula (3) is established.Q1/Q2=A1/A2  (3)
By compensating the pressure in this way, the differential pressures before and after narrowing the first and the second main operation valves have the same value, and the load has no effect. The flow rates Q1 and Q2, which are proportional to the degree of opening A1 and A2 of the first and the second main operation valves, namely, the amount of operation of the left and right operation levers, are supplied to the first and the second hydraulic actuators, and operability when performing complex operations of a plurality of work devices is improved.
(Prior Art 1)
As described above, the hydraulic pressure circuits in Patent literatures 1, 2, and 3 are configured such that pressure compensation by the first and the second pressure compensation valves is released at the same time as the merging/separating valves is switched from the merge position to the separation position, and on the other hand, pressure compensation by the first and the second pressure compensation valves is performed at the same time as the merging/separating valve is switched from the separation position to the merge position.
(Prior Art 2)
In Patent literatures 2 and 3, when the swash plate of one hydraulic pump of the first and the second hydraulic pumps reaches the maximum rotation, and the discharge pressure of the other hydraulic pump has become higher than the discharge pressure of the former hydraulic pump, the merging/separating valve is switched from the separation position to the merge position.
(Prior Art 3)
In Patent literature 3, the merging/separating valve is switched from the separation position to the merge position when a special hydraulic actuator is driven. For example, if one hydraulic pressure motor for traveling is operated, the valve is switched to the separation position, and if the hydraulic actuator for a work device is operated, the valve is switched to the merge position.
Patent literature 1: Japanese Patent Application Laid-open No. 9-217705
Patent literature 2: Japanese Patent Application Laid-open No. 10-82403
Patent literature 3: Japanese Patent Application Laid-open No. 11-218102